DollaGreen

I know this will probably spark much debate, but what is the generally accepted way to determine what you redline is. My origional motor was an L48 with a 5500 rpm redline. I have gone to a non stock block (an inexpensive short block), and top end parts listed in my profile. For all practical purposes, I chose parts (heads, cam) that desktop dynoed at a max of maybe 5000 or 5500 to push it. They fall off power and tq wise after 6k. I dont really wish to push it ever more than that, but wondering what are the determining factors.

45ACP

Redline = the highest the engine can be spun without *damage*, which is determined by the strength of the engine components.


[Modified by 45ACP, 7:54 PM 9/2/2002]

bence13_33

One of the biggest determining factors of how high you can rev your engine is your rod/stroke ratio. The lower the rod/stroke ratio number, the higher the piston speed at any given RPM. To determine the rod/stroke ratio, divide your rod length by your stroke. I.E. a 454 big block 6.135" rod / 4.00" stroke = 1.53 rod stroke ratio.

Obviously forged rotating assembly componants (specifically rods and pistons) will gain you a few extra RPMs of revving capability. You really need to find out where your setup makes the most power. If it's not going to make power past 6000RPMs there's not much sense in revving it any further. Revving a motor to 6000RPMs is not that hard on it. I know the factory redline is 5500RPMs but if your setup will make power up to 6000RPMs I see no harm in revving it that high. A stock L48 motor with decent mileage can easily handle 6000RPMs.

Les

For racing purposes I've heard that you should shift a little past your powerpeak RPM to stay in the fat part of the powerband when you shift. Approx. 300-500 RPMs past peak power is probably a safe bet but your cam manufacturer should be your best source for redline- they know what their cam/lifter/spring combo can pull safely.

GrandSportC3

My understanding of "redline" rpm is the rpm where the engine no longer gains HP which doesn't mean that the engine would get damaged if reving higher... For racing it's recommended to shift 200 RPM past peak HP...

:cheers:

Olivier

Stingy74

My old setup used to make power to just over 6000rpm and the good shifts were at 6700. Those extra few rpms sure do make a difference.

MikeC

While engine components do help determine a redline to some extent, the camshaft is what determines it mostly. The camshaft can be thought of as the brain to your engine...... it tells your engine what to do and when to do it. If you have a stock L-48 cam, the engine can spin up to 6500RPM probably but you are out of your powerband by 5500.

Prime example for me is my 278 energizer is redlines at 6500RPM. So at the drag strip I shifted at 6500RPM. when I started shifting at 5500-5750RPM, I gained 1/2 sec in the quarter mile. my redline of the cam may be 6500 but my power peak is around 5250-5700rpm. So for all intents and purposes, the redline of my car should be 5750.

The only time parts will determine the redline is when you start spinning the engine up higher(6500 and up) and if you are going to spin your engine that fast chances are you'll have good forged parts in there already.

MY63Z11

If you had no tach how would you Know when to shift .With a shift light ?
If you had no shift light?How about seat of the pants? Can you feel the car nose over?When you drive your car hard can you feel when it stop pulling thats when you shift. Any thing past that is red line.But this is OLD SCHOOL
:cheers: I wish my stock camed L48 pulled TO 5500 then I would not have To shift at 4000/4200

Walt


[Modified by MY63Z11, 8:42 PM 9/2/2002]

MIKER

All of these answers sound valid to me.
In summary, my simplified view on this subject is that redline is the maximum rpm that any particular engine combination can spin before mechanical damage occurs OR to the point where power stops increasing. Whichever is less would be the "red line".

Lohkay

Red line is where its dangerous to get mechanical damage. As for what are the limiters...the first one u'll encounter is the cam. If its hydraulic, it'll float at around 6500...maybe 7000 with high perf lifters. Next problem is the crank...the more your rod/bore ratio is the more stress ur putting on the crank thus requiring a stronger one. So its common to reduce the ratio to remove some stress on the crank. Another point is valve lift...the more u have the bigger springs u want...but more springs normally means a shorter lifter/cam life. All an all...a stock setup can handle 6500. a fullproof racing engine can see 10,000. Its really hard to tell what will be the redline for an engine.

Vetterodder

The limiting factor is usually the valvetrain, not just the cam but the lifters, springs, etc.. In some cases, where the valvetrain is optimum and valve control is no longer an issue, the bottom end, especially the ability to provide adequate oiling, can become a major limitation. Except for a few cases, piston speed is not much of an issue in determining an engine's redline.

As others have posted, revving an engine beyond it's peak power will usually benefit acceleration. Though the accelleration rate will drop off once past peak hp, "over revving" will result in starting the next gear at a higher a higher rpm and therefore with more power. An example of this was a car I used to run at the drags. It's power peaked at 5,800 rpm. With it's stock 3.55 gears (which were later changed to 4.10's and 4th gear became more usable) I was running 13.9 @ 109mph if I shifted to 4th but by winding it out and crossing thr traps in 3rd at 6,600 rpm, I would run 13.7 but at 105mph. I did break a valve spring using that tactic (covered under warranty :)). After a cam/lifter/spring replacement with Isky parts, my new redline became 6,800 rpm though it's power peak was now closer to 6,200 rpm and I was running well into the 12's.

MotorHead

Another factor is if your motor was balanced properly when it was built, you can put a 4000-7500 RPM cam in there but if it blows up at 6000 then I would say 5500 RPM was the redline for that particular motor :D

Matt Gruber

with H lifters 5800-6000 according to a recent magazine test.
this is the H lifter redline.
While it is possible to push H lifters higher, go with a solids if you need more rpm.

GrandSportC3

I love my solid cam/lifters!!! I had my 68' a couple of times to 6500 and no problems.... When racing - I usually shift at 6200 - 6400 RPM.

:cheers:

Olivier

ddecart

Interesting. At the strip I've crossed the line in 3rd at something between 6500-7000 rpm (indicated, I need to use my timeslip speec to back-calculate what my REAL rpm was) with no problems. Anyone have the gear ratio for 3rd gear in a Muncie std-ratio 4 speed?

Redline, as I understand it, is a matter of where valve float occurrs and a matter of piston velocities and accelerations combined with the strength of the recirculating assembly (crank & rods). But in all my engine-related engineering books, I have yet to find any specific mention of how to calculate it.

Matt Gruber

red is danger. redline is danger zone.
What is dangerous is an opinion. i had my H 61 to 7000 at least twice. my opinion is it was in the danger zone.
Consider how opinions vary as to maximum temp. in traffic. 230 is ok by me, but many people panic because their opinion is different, or because their car actually boils over by then.
When redline is exceeded it doesn't necessarily blow up, it might be damaged, but we hope not.

Tominator

I learned a lot on this thread, one thing that was left out was fuel starvation. This depends on the fuel setup (intake, carb, clean filter) if your engines starts sounding different like it's dropping off it time to shift.

bb69

Redline is normally based on the RPM where your valvetrain can no longer guarantee that all the parts stay in contact. In other words, how many RPM's before you float valves or toss pushrods. The valvetrain is the limiting factor in almost every case. The way to increase redline is to remove friction and/or mass from your valvetrain. Friction and mass increase the force required to accelerate the parts. These are the things we calculate at work every day. To really calculate your redline, you need the exact geometry of your engine, the part masses, centers of gravity, and coefficients of friction. Also, keep in mind that OEM's will always add a safety factor to the redline to keep people like us from blowing engines up. :D

gkull

You have to build two identicle motors. Then fire one up on a test stand which loads the motor. Slowly increase the rpm untill it blows up. Then place your redline 500 rpm lower than the blowup number on the second one and install it in your car. :cheers:

burners

Gkull, that's exactly my advise. :hat